Conventionally, in some cases, a Mach-Zehnder interferometer is used in an optical modulator that modulates light generated by a light source. In such an optical modulator, a signal electrode and a grounding electrode are provided along parallel optical waveguides. In recent years, diversified optical modulation methods are proposed, and an optical modulator is increasingly provided with a plurality of Mach-Zehnder interferometers. In this case, it is possible to reduce the size of the optical modulator by integrating a plurality of the Mach-Zehnder interferometers in a single chip.
An optical modulator including a plurality of the Mach-Zehnder interferometers can generate a multilevel modulation signal when a plurality of different electrical signals are input. That is, it is possible to perform optical modulation using a multilevel modulation method, such as differential quadrature phase shift keying (DQPSK), when different electrical signals are input from outside to signal electrodes corresponding to the respective Mach-Zehnder interferometers.
An input unit for inputting electrical signals to the optical modulator may be provided with a connector. However, if connectors are provided for respective electrical signals, the size of the optical modulator is increased and a mounting area may be increased. Therefore, the size of the device may be reduced by using a flexible printed circuit board (FPC) having the flexibility, as the input unit for the electrical signals.
Specifically, a plurality of wiring patterns corresponding to a plurality of signal electrodes of an optical modulator are printed on the FPC, and electrical signals output from a driver are input to the optical modulator through the wiring patterns printed on the FPC. On an end of the FPC on the driver side, a pad that is a wide electrode is provided, and the pad is soldered to an electrode on a substrate that transmits the electrical signals output from the driver, so that the FPC and the driver are electrically connected. In contrast, on an end of the FPC on the optical modulator side, for example, a lead pin extending from the optical modulator is soldered to the wiring patterns printed on the FPC, so that the FPC and the optical modulator are electrically connected.
Incidentally, as a wiring pattern printed on the FPC, in some cases, a microstripline may be used in order to transmit an electrical signal at a high frequency such as 10 gigahertz (GHz) or higher. Therefore, on the end of the FPC on the driver side, the wide pad and the microstripline narrower than the pad are connected. In addition, in some cases, a coverlay may be provided on the surface of the FPC in order to cover and protect the microstripline.
Patent Literature 1: Japanese Laid-open Patent Publication No. 2009-258655
However, as for the coverlay, the precision of blanking and bonding is low and tolerance is large. Therefore, even if the coverlay is provided on the surface of the FPC, a part of the microstripline may be exposed. That is, due to a large manufacturing error of the coverlay, the microstripline may be exposed particularly in a portion connected to the pad and this may lead to a disconnection or the like.
To cope with this, it may be possible to increase the size of the coverlay to cover even a part of the pad in order to reinforce a portion where the microstripline is provided on the FPC. However, in this case, the coverlay is sandwiched between the pad and the electrode on the substrate, so that soldering between the pad and the electrode on the substrate may be inhibited. Consequently, a part of the pad is covered by the coverlay and is not soldered, so that impedance mismatch occurs in the connected portion of the FPC and the substrate.
Specifically, the pad of the FPC is designed such that the characteristic impedance reaches 50Ω when the entire pad is soldered to the electrode on the substrate. However, if soldering of a part of the pad is inhibited by the coverlay, it becomes difficult to maintain impedance matching. If impedance mismatch occurs, high-frequency reflection is increased in the connected portion of the substrate and the FPC, and a transmission frequency band is reduced.